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Re: [OM] Tamron 90mm

Subject: Re: [OM] Tamron 90mm
From: "Paul Farrar" <farrar@xxxxxxxxxxxx>
Date: Tue, 7 Aug 2001 09:42:38 -0500 (CDT)
> 
> OK, so now I went and got the 49mm version of the Tamron 90mm 2.5 and it
> really does seem to be extremely well made. At least to me the "fit and
> finish" are great. It may be a little on the large and heavy size but for
> me the feel more than makes up for it. It seems to have a lot of metal in
> it.  
> Anyway I also got the extension tube with it. So now my question is;  is
> there a "light loss" when using the tube and if so how much do you figure
> it would be? Are we talking an f stop or so?
> 
> Thanks again to everyone for their help,
> 
> Jeff Bellin
> Newton, MA

If the lens focuses by moving the whole optical system (and I think yours
does), rather than by changing focal length (called internal focusing) or
a combination of internal focusing and extension (I think the Tamron 90/2.8
and Zuiko 90/2 and 50/2 fall into this category), the focused lens will
have an effective f stop equal to the marked value times 1+m, where m
is the magnification. So at 1/2 life size, f/4 becomes f/6; at 1:1,
f/4 becomes f/8.


Paul

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